The present invention relates to a method for forming a resist pattern on a substrate surface or, more particularly, to a method for forming, on a substrate surface, a resist pattern having an outstandingly high resolving power without the problem of adhering scums after the development treatment. The invention relates also to a scum-remover used in the method.
As is well known, the techniques utilizing a photoresist composition are widely used for the manufacture of various kinds of precision articles such as integrated circuits, photomasks for the processing of integrated circuits, printed circuit boards, printing plates and the like. The principle of the techniques is that the surface of a substrate is coated with a layer of a photoresist composition and the layer is removed pattern-wise to expose the substrate surface bare only on the areas where the substrate surface should be subjected preferentially to subsequent working such as etching, diffusion of a dopant and so on with the pattern-wise photoresist layer as a protecting mask.
The pattern formation using a photoresist composition is primarily performed by a pattern-wise exposure of the photoresist layer to actinic rays, e.g. ultraviolet light, to cause a change in the solubility of the photoresist composition in a liquid called a developer. Relative to the direction of the change in the solubility of the composition in a developer solution caused by the ultraviolet exposure, photoresist compositions are classified into positive-working and negative-working ones. A positive-working photoresist composition is characterized in the increased solubility of the photoresist layer in the developer by the ultraviolet exposure so that the substrate surface is exposed bare after development on the areas exposed pattern-wise to the ultraviolet leaving the photoresist layer on the unexposed areas. A negative-working photoresist composition, on the contrary, is characterized by the decreased solubility of the photoresist layer in the developer by the pattern-wise exposure to ultraviolet so that the pattern-wise ultraviolet exposure and development result in a pattern-wise photoresist layer left on the unexposed areas.
Various kinds of photoresist compositions have been proposed and are practically used in the industrial production including both of the positive-working and negative-working ones. For example, typical and most widely used positive-working photoresist compositions are formulated with an alkali-soluble novolac resin as a film-forming constituent and a photosensitive constituent to promote the increase in the solubility of the composition by a photo-induced decomposition reaction. The developer used for the development of the photoresist layer formed of such a photosensitive composition after pattern-wise ultraviolet exposure is usually an alkaline aqueous solution. No alkali metal compound, however, can be used as the alkalifying agent of such a developer solution, especially, in the processing of semiconductor devices because contamination of the semiconductor material with alkali metal ions is very detrimental against the performance of the semiconductor devices. Accordingly, the developer for the positive-working photoresist compositions usually contains a water-soluble organic base as the alkalifying agent exemplified by quaternary ammonium hydroxides as taught in IBM Technical Disclosure Bulletin, volume 13, No. 7, page 2009 (1970) and choline as taught in U.S. Pat. No. 4,239,661.
One of the requirements for the developer is the selectivity in solubilization of the photoresist composition after pattern-wise ultraviolet exposure, Namely, an ideal developer for a positive-working photoresist composition should dissolve the composition on the ultraviolet-exposed areas as easily as possible while the composition on the unexposed areas should be left absolutely intact by the developer. Further, the cross section of the photoresist layer left pattern-wise after development should desirably be rectangular with the peripheral surfaces vertical to the surface of the substrate while conventional developer solutions usually give a trapezoidal cross section.
The rapid progress of the manufacturing technology of semiconductor integrated circuits in recent years requires finer and finer patterning of the photoresist layer on the substrate surface to obtain a resolving power of so-called submicron order. In particular, the quality and yield of acceptable products are greatly influenced by the performance of the development treatment in the patterned photoresist layer having open areas of a fineness of 1 .mu.m or so or having contact holes. In this regard, conventional developer solutions cannot fully meet the requirements and improvements in the performance of the developer solution have been eagerly desired.
Various attempts and proposals have been hitherto made accordingly in the formulation of the developer solution including a developer solution which is a metal ion-free aqueous solution of a tetraalkyl ammonium hydroxide admixed with a surface active agent of the type of a quaternary ammonium compound such as methyl bis(2-hydroxy)coco-ammonium chloride and trimethyl coco-ammonium chloride as taught in Japanese Patent Kokai 58-9143 and a developer solution obtained by adding a surface active agent or an organic solvent to a conventional developer solution for positive-working photoresist compositions as taught in Japanese Patent Kokai 58-57128.
The above mentioned improvements in the formulation of the developer solution are all based on the mechanism that the admixture of the additives has an effect to increase the wettability of the photoresist layer to the developer solution so as to increase the efficiency of dissolution of the photoresist layer resulting in the improvements in the resolving power and the dimensional accuracy of the photoresist pattern. Though effective to some extent in the desired improvements in the resolving power and dimensional accuracy of the photoresist pattern, the developer solutions by the above mentioned improved formulation still have problems, in particular, when formation of a pattern having very fine open areas or contact holes is desired because scums or thin film residua of the photoresist layer are sometimes left on the areas exposed to the ultraviolet light despite the requirement that the photoresist layer should have been completely dissolved away from such areas resulting in a limited improvement of the resolving power in such fine patterning.
Although the above mentioned scums and film residua on the patterned photoresist layer can be removed by a short post-treatment of the patterned photoresist layer with oxygen plasma or sputtering [see, for example, S. M. Irving, Solid State Technology, volume 14, No. 6, page 47 (1971)], these procedures of post-treatment are practically not feasible because the forms of the resist pattern per se may be damaged unless the post-treatment is performed under well-controlled conditions in addition to the not always satisfactory efficiency for the uniform removal of the scums and film residua, in particular, in an extremely fine pattern with contact holes on the photoresist layer. Accordingly, it is eagerly desired to develop a method for the formation of an extremely fine photoresist pattern on a substrate surface from which the scums and film residua left after the development treatment have been efficiently and completely removed even in the areas of the finest pattern in order to comply with the demand in the semiconductor industry under rapid progress toward increased fineness.